Column packing materials for the applications such as liquid chromatography have conventionally been prepared from silica gel, chemically modified silica gel, polymers, carbon, etc. Porous packing materials are also known and have been prepared from porous silica, chemically modified porous silica, porous polymers, etc.
Calcium phosphate based compounds, in particular synthetic hydroxyapatite represented by (Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2), have the same composition as the inorganic main components of teeth and bones, and, taking advantage of its superior biocompatibility, artificial dental roots or bone prosthetic materials made of synthetic hydroxyapatite have been developed. The biological affinity of hydroxyapatite has been ascribed to the close relationship to biological high polymeric substances such as proteins and sugars.
Attempts have been made for many years to produce packing materials for liquid chromatography from such hydroxyapatite, which is closely related to the living body. In recent years. packing materials that are characterized either by the process of their production or by their shape have been proposed, as described, for example, in Unexamined Published Japanese Patent Application No. 143762/1985. Packing materials based on hydroxyapatite have both cation and anion exchanging properties, and work in a normal-phase mode in analysis of substances such as glycosides. Therefore, hydroxyapatite based packing materials are versatile in that a single column packed with them can be used in a broad range of applications.
One problem with the prior art hydroxyapatite based packing materials, which are not fired, is that they do not have sufficiently high pressure resistance to allow for rapid passage of a mobile phase, with subsequent difficulty in separating large volumes of a sample within a short period of time. As a further problem, the conventional hydroxyapatite based packing materials are highly soluble, so if a mobile phase is caused to flow over an extended period, the surface of the packing material dissolves to cause deterioration of its separating performance (i.e., resolution). Furthermore, the fine particles of hydroxyapatite resulting from the dissolved surface of the packing material will tend to block the passage of the mobile phase and clog the column filter, thereby rendering it no longer usable.
Another major problem with such packing materials prepared from calcium phosphate based compounds is that it is extremely difficult to form granules having a uniform shape and size. When such packing materials are used in liquid chromatography, the number of theoretical plates attainable is small, tailing is prone to occur in chromatograms, and difficulty is encountered in adjustment of the pressure for pumping a mobile phase and controlling its flow rate.
Further, the conventional calcium phosphate based compounds have been chiefly intended for separation of materials such as proteins and enzymes, and in order to attain higher resolution separations, most of the compounds have been used in a porous state having a large surface area. Porous packing materials are also used in gel permeation chromatography (see, for example, Unexamined Published Japanese Patent Application No. 155290/1975). In gel permeation chromatography, however, materials of similar chemical composition are separated in the order of their molecular weight with the higher molecular weight components eluting first.
The present inventors have found that if a porous packing material made of a calcium phosphate based compound is used in separation of saccharides, the separating performance of the packing material is adversely affected by its propensity to cause separation of components in the decreasing order of their molecular weight, as in the case of gel permeation chromatography.
A further problem with calcium phosphate compounds is that they generally have high solubility in acidic solutions, which is a serious problem in various applications and conditions under which packing materials are intended or desired to be used. That is, hydroxyapatite packing shows both cation exchanging ability and anion exchanging ability to proteins, etc., while exhibiting high ability in separation of glycosides in the normal phase mode using acetonitrile and water as an eluent. Owing to such characteristics, a single column packed with hydroxyapatite can be applied to separation of a variety of substances. Since the desired substance can be separated under mild solution conditions, the sample under chromatography is protected from deactivation. Furthermore, the column has a high recovery. Therefore, with developments in the biological industry, hydroxyapatite has been regarded as one of the most promising packings for chromatography. That is, hydroxyapatite is the only one of the apatite compounds which has hitherto been used not only as an implant material, but also as a packing for liquid chromatography, as described in Journal of Liquid Chromatography, Vol. 9(16), pp. 3543-3557 (1986).
However, the hydroxyapatite packing is poor in resistance to dissolution in acidic solutions, sometimes failing to fulfill its function. That is, when an acidic mobile phase is passed through the column packed with hydroxyapatite for a long period of time, crystals of hydroxyapatite are dissolved out and fine crystals released from the surface of packing particles and obstruct the passage of the mobile phase, eventually becoming useless. Therefore the conventional hydroxyapatite packing is not suitable for separation operation in an acidic region. Particularly at a pH of 5.5 or less such packing cannot be used continuously and the range of substances to which it is applicable is naturally limited.
With respect to hydroxyapatite containing fluorine, fluoride uptake by hydroxyapatite has been reported, as described in Colloids and Surfaces, Vol. 13, pp. 137-144 (1985). However, its application to chromatography has not yet been reported or established.